1. Field of the Invention
The present invention relates to a process for preparing unsaturated carboxylic acids from unsaturated aldehydes in the presence of a phosphorus-molybdenum-bismuth-antimony alkali type catalyst.
2. Description of the Prior Art
Many gas phase oxidation processes have been suggested for producing an unsaturated carboxylic acid from an unsaturated aldehyde. These processes include, for example, the preparation of acrylic acid from acrolein over a catalyst consisting of Mo, V, W and silicon (Japanese patent publication No. 12129/1969) and over a catalyst consisting of P, Mo and As (Japanese patent publication No. 19260/1963). Moreover, many processes have also been suggested for the preparation of methacrylic acid as disclosed in U.S. Pat. No. 3,686,294, which employs a P-Mo-As catalyst, Japanese patent publication No. 10773/1973, which employs a catalyst containing Mo and Tl, U.S. Pat. No. 3,795,703, which employs a P, Mo and alkali metal series catalyst and Belgian patent No. 817100, which employs a P, Mo and Sb series catalyst. However, from the viewpoint of industrial suitability, these processes are unsatisfactory with regard to selectivity and lifetime of the catalysts employed.
A need, therefore, continues to exist for a catalyst system which promotes the gas phase oxidation of unsaturated aldehydes to unsaturated carboxylic acids in high selectivity and which possesses good lifetime characteristics.